How do you know if your workout is working? If you're no longer motivated, you hurt, or you've lost your momentum, those may be signs that you need a change.
Muscle soreness or if you're hurting a lot could be a sign of over training. Be sure to schedule some days of rest in between your work out days. You need to be sure to give your muscles time to repair themselves. Are you only doing one type of workout? Then try alternating between cardio and weight training. If you're too tired to work out you may just need to get a little more sleep. Nobody can get in a good workout when they haven't gotten enough sleep. Sleep is important.
If your workout seems easy, then you're making progress. But that also means you need to change things up. Challenge yourself a little more. If you dread working out, then it may be time to change things by trying a new routine, switching gyms or even listening to different music while working out. And don't just do your favorite exercises - you have to challenge yourself and work different muscles.
Have you hit a plateau? Over time your muscles adapt, so you need to switch gears by adding more weight, intensity or frequency.
Take time to evaluate your work outs every 6 weeks or so. Don't let yourself become bored with your exercise routines. Challenge yourself, try new things, get proper rest and keep up the good work!